There's no real argument against it being legal, IMO. Most people against it will say that it victimizes people, but that's what it being illegal does. Prostitutes are left with no legal recourse when they are stiffed, assaulted or otherwise victimized during the practice of their trade. Furthermore, illegality prevents any sort of regulation that would aid in the safety and quality of the trade.
Making it legal would open it up to legitimate employers who would help the trade become safer.
I mean, sure, it being illegal probably reduces the incidence of it, but I'd rather have a much safer market of prostitution instead of trying to reduce that market by some amount.